Ethylene oxide is a substantial item of chemical commerce having utility both as a sterilization agent and as a fumigant, but primarily as a starting material in the manufacture of a diverse array of products such as anti-freeze, cosmetics, lubricants, plastics and surfactants. The major process for manufacture of ethylene oxide is by the silver-catalyzed oxidation of ethylene. Typically, the stream exiting the reactor comprises small quantities of ethylene oxide (for example 0.5 to 5 mol %) together with large amounts of residual gases including unconverted ethylene and oxygen as well as appreciable quantities of carbon dioxide, low molecular weight hydrocarbons and inert gases such as nitrogen. Customarily, the ethylene oxide product is recovered from the residual gases by absorption in water followed by processing of the ethylene oxide-fat absorbent in a variety of ways including fractionation, scrubbing, stripping and the like.
An ethylene oxide plant may also be the site of production of monoethylene glycol derived by hydration of the product ethylene oxide. In such a plant, the product ethylene oxide, having been isolated by water absorption, is then typically hydrated in a separate reactor.
Variations of this process have been proposed in the past. GB-A-498,119 for example is an early proposal where the hydration may be performed thermally or, preferably, using an acidic catalyst which is added to the water absorbent. The glycol forms on heating the absorbent solution after the ethylene oxide is absorbed and the resulting solution is recycled as liquid absorbent until the glycol content is relatively high and is recovered.
EP-A2-776890 proposes another variant where ethylene oxide absorbed in a solution containing ethylene carbonate and ethylene glycol is reacted with carbon dioxide to the carbonate form and hydrolyzed with a separate catalyst. The Example shows that treatment of an ethylene oxidation product stream, containing 3 mol % ethylene oxide, left 100 ppm in the residual gases, following the particular absorption step and conditions used.
DE-A1-19843721 uses ethylene oxide wash water as absorbent prior to the hydration step. The Example shows absorption in wash-water of ethylene oxide from a gas mixture containing 2.7 mol % ethylene oxide, leaving 10 ppm ethylene oxide to be lost in the residual gases.
The residual gases that remain after recovery of the bulk ethylene oxide product are recycled to the ethylene oxidation reactor. Customarily, a small bleed stream is withdrawn from the recycled gases to prevent build-up of impurities such as argon, ethane or nitrogen in the recycle gas loop. A side stream, being part or all of the recycle gas, is usually scrubbed with an aqueous carbon dioxide (CO2) absorbent for removal of excess CO2 which is subsequently stripped from the absorbent and typically is vented, or if desired, recovered for use or sale as a by-product.
A problem arises, particularly in manufacturing plants of large capacity, in that, during scrubbing of the recycle gas side stream, small amounts of hydrocarbon are dissolved and/or entrained in the CO2 absorbent and ultimately vented with the carbon dioxide.
U.S. Pat. No. 3,867,113 discloses a process improvement which is now conventional in EO processes, whereby the CO2-fat absorbent obtained by contacting a sidestream from the recycle stream with a CO2 absorbent is flashed to form a hydrocarbon-containing vapor stream and a hydrocarbon-lean fat absorbent, and the fat absorbent is stripped to produce a CO2 stream substantially free of hydrocarbon which is suitable for venting, or use.
Nevertheless, in conventional EO plants, despite flashing the fat absorbate, a small amount of residual EO remains in the vent gas from the CO2 stripper. In order to keep vented EO as small as possible for environmental reasons, the vent gas can therefore, conventionally, be subject to incineration. Such incineration can, for example, occur within a catalytic incinerator whereby one or more catalyst beds are heated to high temperatures (in ranges from approximately 300° C. to 800° C.) and various heat exchange mechanisms are incorporated to minimize energy loss and improve efficiency. This, therefore, presents a convenient solution which is effective in operation and, as an “end-of-pipe” process, requires minimal alteration to the existing process line up.
Being operated at high temperatures and requiring the need for catalyst will, however, inevitably mean periods of time in which the incinerator cannot function due to the need for maintenance, repair, replacement of catalyst, etc. Where environmental contamination is measured on an average basis, say as EO vented per week, such incinerator-downtimes are negligible, but increasingly, environmental concerns require constant monitoring of EO venting with hourly measurement of contamination, and this means that the conventional incinerator system may not comply with environmental regulations all of the time.
There is therefore a need to provide for an effective reduction of EO in CO2 vent gas from an EO plant, with or without the use of additional incineration, in which reduction is constantly maintained.
The concentration of EO in vent gas is moreover dependent on a number of factors, including the percent of recycle stream which is treated as a sidestream, and the operating conditions of the CO2 absorber and EO absorber. A further need is, therefore, to provide a system for independent control of EO in CO2 vent gas from an EO plant, whereby the level of carbon dioxide to be vented can be varied without compromising the reduction in EO emission.